Cork is the material which, by virtue of its properties of corrosion resistance, resilient return after compression—which enables the closure to form a good seal—and impermeability to gases, is normally used for the manufacture of stoppers for medium-quality and good-quality wine bottles.
However, cork is a material of natural origin and, as such, is subject to some disadvantages which render its use ever less desirable. In the first place, its high cost, particularly for good-quality cork stoppers, combined with the fact that it is difficult to obtain, tend to confine its use ever more to good-quality wines or liquors. In the second place, precisely because cork is a natural material, its properties are subject to considerable variability. As a result, a considerable percentage of wine bottles have to be discarded owing to defects of closure such as leakage of liquid or the characteristic and unpleasant “corked flavour” conferred on the wine by a poor-quality cork stopper. A poor-quality cork stopper also tends to crumble when removed, thus contaminating the contents of the bottle.
Wine stoppers made of synthetic material are already known. They may be made of various materials, particularly of elastomers such as expanded polyethylene or copolymers thereof, polystyrene and copolymers thereof, EVA, polyurethanes, polypropylenes, and so on. However, although known stoppers have some characteristics which render them similar to conventional cork stoppers, such as good resilient return after compression and good impermeability to gases, they still have some disadvantages which may make their use inadvisable.
In particular, United States patent U.S. Pat. No. 5,496,862 in the name of Supreme Corq describes stoppers made of a thermoplastic elastomer, that is, styrene-ethylene-butylene-styrene (SEBS), in which the expansion of the material is brought about by the addition of a chemical expanding agent in a quantity of between approximately 3% and 5% by weight, calculated relative to the weight of the composition. Although the expanding agent generates gas during the expansion of the polymer and is therefore partially eliminated from the final product, a certain amount of expanding agent inevitably remains in the stopper in unreacted form or as a degradation product. This is certainly disadvantageous since it may lead to contamination of the contents of the bottle.
The same patent cited above also describes the possibility of printing captions on the stopper with ink. If these inscriptions are produced on the surface of the stopper which comes into contact with the neck of the bottle or with the wine—which practice is ever more widespread in the field—once again, the contents of the bottle may be contaminated by the stopper.
There is therefore a considerable demand for container closures which are made of synthetic material but which have characteristics similar to those of a cork stopper—that is, good resilient return after compression, good impermeability to gases, and the ability to be removed with corkscrews in conventional manner—without, however, having the disadvantages of cork closures or of those made of plastics material according to the prior art, in particular, the possible transfer of contaminating materials or substances to the contents of the bottle.